Ukrainian President Petro Poroshenko, center, poses with European Union foreign policy chief Federica Mogherini, left, and European Council President Donald Tusk as they pose prior to a meeting at the Europa building in Brussels, Wednesday, Dec. 12, 2018. (Aris Oikonomou, Pool Photo via AP)

Pool Photo via AP

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Ukrainian President Petro Poroshenko, center, joins hands with European Union foreign policy chief Federica Mogherini, left, and European Council President Donald Tusk as they pose for a photo prior to a meeting at the Europa building in Brussels, Wednesday, Dec. 12, 2018. (Aris Oikonomou, Pool Photo via AP)

Pool Photo via AP

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Ukrainian President Petro Poroshenko, center, poses with European Union foreign policy chief Federica Mogherini, left, and European Council President Donald Tusk prior to a meeting at the Europa building in Brussels, Wednesday, Dec. 12, 2018. (Aris Oikonomou, Pool Photo via AP)

Pool Photo via AP

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Ukrainian President Petro Poroshenko, center, joins hands with European Union foreign policy chief Federica Mogherini, left, and European Council President Donald Tusk as they pose prior to a meeting at the Europa building in Brussels, Wednesday, Dec. 12, 2018. (Aris Oikonomou, Pool Photo via AP)

Pool Photo via AP

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High Representative for Foreign Affairs and Security Policy Federica Mogherini (L) President of the European Council, Donald Tusk (R) and Ukrainian President Petro Poroshenko pose at the European Council headquarter on December 12, 2018 in Brussels ahead of a meeting. (Photo by Aris Oikonomou / AFP)

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High Representative for Foreign Affairs and Security Policy Federica Mogherini (L) President of the European Council, Donald Tusk (R) and Ukrainian President Petro Poroshenko shake hands for a shooting photo at the European Council headquarter on December 12, 2018 in Brussels ahead of a meeting. (Photo by Aris Oikonomou / AFP)

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President of the European Council, Donald Tusk (R) shakes hands as he receives Ukrainian President Petro Poroshenko (L) at the European Council headquarter on December 12, 2018 in Brussels ahead of a meeting. (Photo by Aris Oikonomou / AFP)

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President of the European Council, Donald Tusk (R) receives Ukrainian President Petro Poroshenko (L) at the European Council headquarter on December 12, 2018 in Brussels ahead of a meeting. (Photo by Aris Oikonomou / AFP)

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President of the European Council, Donald Tusk (R) receives Ukrainian President Petro Poroshenko (L) at the European Council headquarter on December 12, 2018 in Brussels ahead of a meeting. (Photo by Aris Oikonomou / AFP)

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President of the European Council Donald Tusk (R) shakes hands as he receives Ukrainian President Petro Poroshenko (L) at the European Council headquarter on December 12, 2018 in Brussels ahead of a meeting. (Photo by Aris Oikonomou / AFP)